Process for preventing short-circuiting by an electrode in an electric arc furnace having a liquid melt therein



Sept. 24, VOGT L PROCESS FOR PREVENTING SHORT-CIRCUITING BY AN ELECTRODE IN AN ELECTRIC ARC FURNACE HAVING A LIQUID MELT THEREIN Filed June 20. 1960 INVENTOR. A040 M067 V/l/ TE W/[BKE United States Patent 3,104,939 PROCESS FOR PREVENTING SHORT-CIRCUETHNG BY AN ELECTRODE IN AN ELECTRIC ARC FURNACE HAVING A LIQUID MELT THEREIN Adolf Vogt, Munich, and Giinter Wiebke, Grefrath, near Cologne, Germany, assignors to Elektroschmelzwerlr Kempten Gesellschaft mit beschrtinktcr Haftung, Munich, Germany Filed June 20, 1960, Ser. No. 37,088 Claims priority, application Germany June 24, 1959 Claims. (Cl. 23-1) The present invention relates to a process for making elements or compounds thereof in electric arc-type furnaces. More in particular, the present invention relates to a process for producing in electric arc-type furnaces such elements or compounds thereof the vapor phase of which has a poor electric conductivity in the electric are.

It is known to produce elements or compounds in electric arc type furnaces having one or several electrodes. In processes where both a metal and a slag are formed the electrodesare positioned in the mass of slag or in the charge in order to reduce the radiation of heat and obtain a better utilization of thermal energy. According to the load applied, the electric current flows either from the electrode through the slag to the metal bath formed underneath the slag, or from a first electrode through the slag to another electrode.

If materials are processed yielding a bottom layer of liquid metal and vapor having an excellent electric conductivity, but not yielding any slag, the electric current passes through the layer of vapor in the arc to the bottom material.

Considerable problems are, however, encountered when processing materials yielding a bottom material comprising a liquid metal bath and, thereabove, a vapor layer, which vapor has a poor electric conductivity in the electric arc. In view of this poor conductivity it becomes necessary to position the electrode very close to the liquid bath in order to obtain a current flow. In practice, the electrode will frequently come int-o direct contact with vthe bath, particularly in view of the turbulence of the melt resulting from the evolution of gas. As a consequence of such contact, the current flow is short-circuited and a sud-den current increase is produced which reaches such excessive values that it becomes necessary to remove the electrode from its position close to the bath.

it will be apparent that, under these circumstances, it is not possible to carry on a continuous process. Furthermore, the frequent interruptions prevent the application of the optimum current load and instead of a substantial liquid metal bath columns of sol-id material are obtained. It is, therefore, not possible to continuously tap the desired material from the furnace, as such a continuous tapping requires a substantial liquid bath of metal continuously and constantly produced in the furnace. For the above mentioned reasons such a continuous and constant production is not possible under the known art processes.

Furthermore, it has been found that the known art processes are disadvantageous inasmuch as they operate with a finely ground or pulverized charge. This charge tends to agglomerate in the furnace which first prevents the gases produced in the furnace from escaping or to burn regularly. Then, after a short while, the pressure of the gases forms various channels in the charge in the furnace, and the gases escape through these channels in a wild, turbulent rush, pushing upwardly the charge and preventing it from gradually falling downwardly as the lowermost portion of the charge is molten, and even blowing away the powderous particles of the charge.

With the foregoing in mind it is an object of the pres- 2 out invention to provide a method for producing elements or compounds in an arc-type furnace, whose vapor phase in the arc has a poor electric conductivity, which enables the application of an optimum current load and which is susceptible to continuous and efiicient operation.

It is another object of the present invention to provide a method for producing elements or compounds in an arc-type furnace, whose vapor phase in the arc has a poor electric conductivity which enables the furnace gases to escape or burn regularly without disturbing the charge and its continuous downward movement in the furnace.

Further objects .and advantages will become apparent as the description proceeds.

The objects are achieved by the process of the invention which is characterized by a first feature of adding to the charge an kali or alkaline earth metal, or a compound thereof, and by a second feature of forming the charge into lumps of a size of between 10 to 300 millimeters and feeding the charge to the furnace in the form of such lumps.

it is also possible to select a charge which already contains one of the aforementioned substances, thus making it unnecessary to add the same to the charge prior to forming the lumps and feeding these to the furnace.

It will be noted that the presence of one or several of the aforementioned substances will increase the conductivity of the charge, thereby making it unnecessary to approach the electrode so close to the bath that a shortcircuit is established, resulting in the disadvantages explained further above.

It will be further noted that the charge consisting of lumps will allow for the regular and undisturbing passage of the furnace gases, thereby obviating the above-described disadvantages.

The afore-mentioned substances should be added to or should be present in the charge in an effective and, at the same time, economical amount, which generally is comprised in the range from 0.0 1 to 0.3% of the charge, and which preferably should not exceed 1.4% of the charge. By varying the percentage, with other conditions remaining equal, the conductivity and, hence, the temperature in the furnace can be varied, thereby obtaining a substance of different quantitative composition, as, e.g., AlB instead of AIB By varying the percentage in the course of one operation a mixture of such different final substances, e.g. a mixture of AlB can be obtained.

According to the second basic feature of the invention, the charge, containing the basic raw material needed for the desired element or compound, and the above-described additional substances, is supplied to the furnace in the form of lumps. These lumps can be formed, e.g., by first grinding the raw materials, mixing the same, and adding one of the above-described substances (unless a charge is selected already containing one of the substances). Water can be added so as to obtain a mass of pasty consistency. Thereafter, the mass is filled into forms and dried, thereby obtaining a number of cakes. It will be noted that the paste should be thoroughly dried to get cakes of a low water content, which will later on result in a saving of thermal energy in the electric furnace. Finally, the cakes are broken-up into lumps of a determined average size, within the above indicated size limits.

As the present invention eliminates the undesirable interruptions necessary with the known processes and enables a continuous operation, the charge can be, and preferably is, fed continuously to the furnace.

To facilitate molding of the paste a bonding agent sucn as a dextrin can be added.

The process of the present invention is susceptible to produce elements'or compounds thereof the vapor phase 3 of which has a poor electric conductivity, 'e.g., carbide, such as silicon carbide, boron carbide; oxides having a high melting point, i.e. above 1200 C., e.g. silicon dioxide; borides, such as titanium boride, aluminum boride; silicides and metals or metalloids, respectively, of the third and the fourth group of the periodic system.' It is also possible to produce, with the rnethod of the invention,

. mixtures of compounds, such as SiC and boron carbide,

or alloys.

Our process is carried out in a conventional type electric arc furnace, a suit-able form of which is shown in the accompanying drawing in which 3 represent the walls of a conventional electric arc furnace, resting on a suitable conventionalbase represented by 4 and'5'. Conventional electrodes are represented by 1, supported by conventional electrode supports 2 which permit the electrodes to be raised or lowered. FIGURE 7 represents the reaction mixture before fusion, and FIGURE 8 represents molten reaction mixture resulting from the passage of the electric arcthrough the reaction mixture.

The invention will be further explained with reference to the following examples which are, of course, not to be considered as limitative of the scope of the invention or its applicability. Example I 25 kilograms of finely ground oil coke, having a carbon content of 88%, and 60 liters of water containing 60 grams of caustic soda, so as to obtain a pasty consistency. This pasty mixture is brought into flat cups and the circulating air, thereby drying the pasty mixture and obtaining a number of flat cakes. 'Ihe cakes are removed from the cups and are then broken up to form a number of approximately 4 to 8 centimeters. charge which is fed to the furnace. The electrodes are entirely covered with this charge. At a voltage of 80 and a voltage of 80 to 2000 amperes without encountering 1 ,cups are then placed into a vapor drying chamber with lumps, the greatest diameter of which is in the range from 7 These lumps form the 2000 amperes the furnace operates very calmly and regularly. The gas produced in the furnace burns entirely at the surface of the charge and no blow elfect is observed. A bottom material of 13 C is formed below the electrodes as a homogeneous integral melt.

In a large scale production lumps the greatest diameter of which is up to 30" centimeters are continuously molten many) and 10 grams of caustic soda.

gram of caustic soda in the form of lumps.

charge.

Example III In a furnace, 'as described in Example I, a charge is prepared in the manner described in the first example but consisting of 67.5 kilograms of powderous titanium dioxide, 36 kilograms of finely ground oil coke, 60 liters of water, 0.5 kilogram tylose (a water-soluble cellulose ether sold by the firm of Kalle, Wiesbaden, West Ger- The cakes which are formed and dried are broken up in lumps of 4 to 8 centimeters. At a voltage of 60 and 2 /2 hours of operation, '100 kilograms of the charge are molten and a block of titanium carbide containing 19.8 of carbon is obtained without encountering the disadvantageous effects of Example II.

Example 1V Example V Example IV is repeated with the exception that a charge consists of 90 kilograms of boric acid, 4.8 kilograms of alumina, 32 kilograms of oil coke and 0.5 kilo- Boron aluminum is obtained in a continuous operation and at the disadvantageous effects of Example II.

5 Example VI Example V is repeated, however, a greater percentage of caustic'soda is added, thereby lowering the operating temperature in the furnace and obtaining an aluminum boride of higher aluminum content.

Example VII V In the furnace of Example I and at a voltage of 80 a charge is processed containing 60 parts by weight of sand and 40 parts by weightof ground oil coke. After 20 minutes a connection is formed between the electrodes conducting electric current and extinguishing the arc.

The operating furnace is stopped and examined and a graphite bridge between the electrodes is discovered,

and the bath is tapped in successive periods of 2 to 4 hours.

Example I] ing the electrodes, a short-circuit is established with the bottom material and the operation has to be interrupted. In spite of the small load after a period of only 20 minutes the powderous charge agglomerates and the furnace gases are. prevented from escaping regularly. Briefly thereafter, various passages are formed in the agglomer- A ated powder and the furnace gases escape in a wild, turbulent flow.

Furthermore, in contrast with the lumps in Example I, the powderous rnass does not gradually fall downwardly in the furnace as thelowermost portion of the charge demonstrating that the furnace operated as a resistance furnace.

- Example VIII Example VII is repeated with the exception that the charge, prior to being fed intothe furnace, is mixed with 15. parts by weight of water glass and 15 parts by weight of water, is then dried, whereupon the cakes are broken Example I is repeated with the exception that the charge contains, by weight, 100 parts of boric acid, 18

parts of sand, 44 parts of soot and 0.5 part of caustic soda. Ata voltageof and 2500 amperes the furnace operates very calmly and an integral melt is obtained containing 23.5% C., 28.5% Si, 47.8% B.

Example X 7 Example I is repeated with the exception that the charge contains 200 kilograms of quartz sand, 93 kilograms of oil coke, 40 liters of sodium water glass of 38 B. and 40 liters of water. Silicon is obtained after a ,very regular, continuous and calm operation.

Example XI Example I is repeated with the exception that the charge consists of quartz of a grain size of 15 to 25 millimeters, and 0.2% solid soda and that the voltage is 80 and the current 1500 amperes. In a calm operation of the furnace a block of molten quartz is formed below the electrodes.

It will be understood that this invention is susceptible to further modification and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What is claimed is:

1. In a process for preventing short-circuiting when one or more electrodes approach the layer of a liquid mel-t formed in an electric-arc-type furnace under reaction conditions in which the vapor phase of the product formed in the said electric arc has low electric conductivity, the step which consists in feeding into said electric arc-type furnace under conditions a reaction mixture in lump form of a size between about and 300 mm. containing 0.01 to 1.2% by weight of a material selected from the group consisting of inorganic alkali metal compounds, inorganic alkaline earth metal compounds, and inorganic magnesium compounds.

2. The process of claim 1 wherein the said reaction mixture contains from 0.01 to 0.3% by weight of a material selected from the group consisting of inorganic alkali metal compounds, inorganic alkaline earth metal compounds, and inorganic magnesium compounds.

3. The process of claim 1 wherein said reaction mixture comprises an intimate mixture of a material having low electrical conductivity in the vapor phase in the electric arc, selected from Groups Ill and IV of Mendeleils Table, and 0.01 to 1.2% by weight of a material selected from the group consisting of inorganic alkali metal compounds, inorganic alkaline earth metal compounds, and inorganic magnesium compounds.

4. The process of claim 1 wherein said reaction mixture is in lump form of a size of between about 10 and 300 mm. and comprises an intimate mixture of a material having low electrical conductivity in the vapor phase in the electric arc, selected from Groups Ill and IV of Mendeleffs Table, and from 0.01 to 0.3%, by weight, of a material selected from the group consisting of inorganic alkali metal compounds, inorganic alkaline earth metal compounds, and inorganic magnesium compounds.

5. The process of claim 1 wherein the reaction mix- 8 ture fed into the electric arc furnace under reaction conditions comprises an intimate mixture of boric acid, carbon, and caustic soda, and the resulting reaction product is boron carbide.

6. The process of claim 1 wherein the reaction mixture fed into the electric furnace under reaction conditions comprises an intimate mixture of titanium dioxide, carbon, and caustic soda, and the resulting reaction product is titanium carbide. I

7. The process of claim 1 wherein the reaction mixture fed into the electric arc furnace under reaction conditions comprises an intimate mixture of boric acid, alumina, carbon, and caustic soda.

8. The process of claim 1 wherein the reaction mixture fed into the electric arc furnace under reaction conditions comprises an intimate mixture of titanium dioxide, boric acid, and magnesium oxide.

9. The process of claim 1 wherein the reaction mixfed int-o the electric arc furnace under reaction conditions comprises an intimate mixture of sand, carbon, and Water glass.

10. lrocess for the production of carbides, borides and silicides of materials having low electrical conductivity in the vapor phase, selected from Groups III and IV of Mendeleiis Table, which comprises feeding into an electric arc furnace under reaction conditions in lump form of a size between about 10 and 300 mm. an intimate mixture of materials having low electrical conductivity in the gaseous form, selected from Groups Ill and IV of Mendeleils Table, and 0.01 to 1.2% by weight of a material selected from the group consisting of inorganic alkali metal compounds, inorganic alkaline earth metal compounds, and inorganic magnesium compounds.

References Cited in the file of this patent UNITED STATES PATENTS 628,806 Harry July 11, 1899 648,463 Knaur May 1, 1900 690,319 Roberts Dec. 31, 1901 842,273 Tone Jan. 29, 1907 OTHER REFERENCES McPherson and Henderson book: A Course in General Chemistry, pages 410, 547, 560, 562, 570, 573. (Copy in Congressional Library and personal copy in Division 59, third edition, 1927.) 

1. IN A PROCESS FOR PREVENTING SHORT-CIRCUITING WHEN ONE IOR MORE ELECTRODES APPROACH THE LAYER OF A LIQUID MELT FORMED IN AN ELECTRIC-ARC-TYPE FURNACE UNDER REACTION CONDITIONS IN WHICH THE VAPOR PHASE OF THE PRODUCT FORMED IN THE SAID ELECTRIC ARC HAS LOW ELECTRIC CONDUCTIVITY, THE STEP WHICH CONSISTS IN FEEDING INTO SAID ELECTRTRIC ARC-TYPE FURNACE UNDER CONDITIONS A REACTION MIXTURE IN LUMP FORM OF A SIZE BETWEEN ABOUT 10 AND 300 MM. CONTAINING 0.01 TO 1.2% BY WEIGHT OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF INORGANIC ALKALI METAL COMPOUNDS, INORGANIC ALKALINE EARTH METAL COMPOUNDS, AND INORGANIC MAGNESIUM COMPOUNDS. 